1. Field of the Invention
This invention relates to a welding process using an eutectic alloy wire suitable for welding a superlow-temperature steel such as 9% nickel steel.
2. Description of the Prior Art
9% nickel steel is a high tensile steel which may be used at a superlow temperature up to -196.degree. C. The tensile strength of the 9% nickel steel is defined to be on the order of 70.3-84.4 kg/mm.sup.2 according to the ASTM standard, A353 (NNT material) and A553 (QT material), and the yield point (0.2% yield strength) higher than 52.7 kg/mm.sup.2 and higher than 59.8 kg/mm.sup.2 according to A353 and A553. The ASTM standard also requires that the impact value thereof be greater than 3.5 kg-m at -196.degree. C. A further requirement of the ASTM standard, case 1308-5, when construction of a building is accomplished by welding the 9% nickel steel, is that the tensile strength of a joint including a base metal material be higher than 66.8 kg/mm.sup.2 and lower than that of the base metal material per se in order to assure joint performances when annealing is not carried out as a welding condition for the removal of stress.
In recent years, however, there has been a strong desire for the development of joints of tensile strength well above the standard value as defined by the case 1308-5 and welding materials of a strength not less than that of the base metal material for increasing stress at time of design for welding. As is obvious from the ASTM standard, proper strength and low-temperature toughness of the 9% nickel steel is obtainable from heat treatment but in the case of large building construction, for example, a storage tank, such heat treatment is substantially impossible after completion of the construction. To this end the construction is made serviceable for welding conditionings.
While it is most desirable to use a welding wire whose composition is identical to that of the base material for welding the 9% nickel steel, high nickel alloy wires as defined by the AWSA standard, 5.11 ENiCrFe.1-3, etc., are very often actually used for welding because there are difficulties in obtaining stable low-temperature toughness of the 9% nickel steel wire. While joints made through use of the high nickel welding wire exhibit excellent toughness at a temperature of -196.degree. C. after welding, they undergo very small tensile strength (particularly, 0.2% yield strength) as compared to that of the base metal material. No matter when the 9% nickel steel or a 70 kg/mm.sup.2 high tensile steel is used, the strength of the joints is low so that weld design stress should be low and the overall construction welded should be thick. The conventional welding method fails to take full advantage of the strength property of the 9% nickel steel and, in fact, suffers from twofold uneconomical expenditures, an increased thickness of the construction welded and an increased amount of expensive high nickel alloy welding wire consumed. Welding by the high nickel alloy is further disadvantageous due to the experiencing of hot cracks and thermal fatigue due to a difference between the coefficients of thermal expansion, thus requiring laborsome welding procedures.
For these reasons 9% nickel steel is severely limited in application while showing excellent performances as a superlow-temperature steel.